The present invention relates to electroluminescent devices, and more particularly, to the fabrication of organic light emitting diodes having more predictable colors.
Organic polymer-based electroluminescent devices (OLEDs) have the potential for providing inexpensive alternatives to alpha-numeric displays and x-y addressable displays. Typically, an OLED consists of a transparent substrate coated with a transparent conducting material, such as Indium Tin oxide (ITO), one to five organic layers and a cathode made by evaporation or sputtering a metal of low work function characteristics, such as Ca or Mg. The organic layers are chosen so as to provide charge injection and transport from both electrodes to the electroluminescent organic layer (EL) where charges recombine emitting light. Usually there are one or two organic hole transport layers (HTL) between the ITO and EL, as well as one or two electron injection and transporting layers (EL) between the cathode and the EL.
The output spectrum generated by such devices often differs significantly from that predicted from the chemical structure of the material used for the EL layer. When the device output spectrum differs from the design spectrum, the designer typically adjusts the composition of the EL material and tries again. This hit or miss approach substantially increases the design cost.
In full color pixelated displays, the designer must provide three colors of pixels, and hence, the problems of unpredictable color output are increased. In addition, the color pixels must provide similar light outputs for any given drive current.
Broadly, it is the object of the present invention to provide an improved OLED.
It is a further object of the present invention to provide an OLED that has a predictable color output.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is an OLED for emitting light at a predetermined peak wavelength, xcex. The OLED includes an anode layer, a cathode layer and an electroluminescent layer constructed from an organic light emitting compound that generates light, including light having a wavelength in a band around xcex, by the recombination of holes and electrons. The electroluminescent layer is electrically connected to the anode and the cathode layers and is located between the anode and cathode layers. The OLED includes first and second reflectors, displaced from one another. The anode or cathode layers can serve as one of the reflectors. A spacer layer constructed from a material that is transparent at xcex is included between the reflectors and has a thickness that is adjusted such that the optical path length between the first and second reflectors is equal to N xcex/2, where N is a positive integer. The spacer layer may include a hole transport material located between the electroluminescent layer and the anode. The spacer layer may also be constructed from a layer that is located between the anode or cathode layers and one of the reflectors. If the spacer layer is located between the anode and cathode layers, the spacer layer is constructed from an electrically conducting material.